Reduction of spurious responses in radio reception



March 9, 194 D. E. FOSTER ET AL.

REDUCTION OF' SPURIOUS RESPONSES IN RADIO RECEPTION 3 Sheets-Sheet 1 Filed Dec. 7, 1940 INVENTORS ATTORNEY March 9, 1943. FOSTER HAL 2,313,635

REDUCTION OF SPURIOUS RESPONSES IN RADIO RECEPTION Filed Dec. 7, 1940 3 Sheets-Sheet 2 w a audio/"o.

nabs-From 40 Channels sew ImZ flarmora'o Under ml and 0 cdZaZZ L mar/11mm? 9. Oscdlalbrflarmrao Under Osa'daZbrflr'gker rm): 7202a fregm y Franz 2 7'0 50/710. 07 -6 mm 2m W emf m m% J w M 0 m M Patented Mar. 9, 1943 REDUCTION OF SPURIOUS RESPONSES IN RADIO RECEPTION Dudley E. Foster, South Orange, N. J., and John A. Rankin, Port Washington, N. Y., assignors to Radio Corporation of America, a corporation of Delaware Application December '7, 1940, Serial No. 369,040

3 Claims.

Our present invention generally relates to reduction of spurious responses in superheterodyne type receivers, and more particularly to a novel method of, and means for, substantially reducing various spurious responses in superheterodyne receivers of the frequency modulation type.

In the following specification reference is made to a superheterodyne type receiver to receive frequency modulated carrier waves. Those skilled in the art are fully aware that a method by which spurious responses in a specific type superheterodyne receiver are reduced will apply also to any type of superheterodyne receiver. Frequency modulation receivers are subject to spurious responses as are amplitude modulation receivers, and because of the probable frequency modulation transmitter locations, the spurious responses are likely to be worse than on the present amplitude modulation broadcast band of 550-1700 kilocycles. The superheterodyne type receiver is used almost universally today because of its great advantage in sensitivity and selectivity, but it is subject to spurious responses. The selectivity requirements in the 42 to 50 megacycle frequency modulation band are so severe that the use of the superheterodyne is even more necessary there than in the standard amplitude modulation broadcast band. Stations may be assigned to alternate channels in the frequency modulation band in a given locality. Since the channels are 200 kilocycles wide this permits assignments 400 kilocycles apart. At the mean frequency modulation band frequency of 46 megacycles a separation of 400 kilocycles is the same percent separation as kilocycles at 1150 kilocycles so that selectivity becomes a prime consideration.

Since th service area of a frequency modulation transmitter is essentially limited by the horizon, transmitter antennae will have as great a height as possible. In large cities this means that the tendency will be to place the transmitter antenna on the tallest building available. Since tall buildings are generally in the center of the city, it is likely that many frequency modulation transmitters will be located in close proximity to each other near the center of population of the area. With transmitters so located, high field intensities will occur at many receiving locations. Th s is a condition which favors generation of spurious responses. For example, a transmitter on approximately 46 megacycles with an antenna 1990 feet high which will deliver a field of 1 millivolt per meter at 30 miles, will have a field in tensity of about 0.75 volt per meter at one mile, and about 0.15 volt per meter at 3 miles. With a lower antenna and increased power to give the same field intensity at 39 miles, the field intensity close to the transmitter will be even greater.

Spurious responses may be of several types, some types being unique to superheterodyne receivers, other types existing in any type receiver. The types of spurious responses which may exist in frequency modulation receivers are:

. Image response.

. Direct intermediate frequency response.

. Response from two stations separated by the intermediate frequency.

monies. 5. Half intermediate frequency image response. 6. Cross modulation.

Cross modulation, Whether within the receiver or of the external type, does not depend upon the type of receiver. The other forms of spurious responses occur only in superheterodynes.

There will now be given a discussion of the nature of these undesirable responses with a View to showing the utility of the invention. Image response in frequency modulation receivers is similar in nature to that in amplitude modulation receivers. Since the band extent is only 8 megacycles (42-43 megacycles educational, and 43-50 megacycles commercial broadcasting) any intermediate frequency over 4 megacycles will prevent image response from frequency modulation stations. Consideration must also be given to image response from other radio services, but except for television transmitters and possibly amateur and police transmitters, the location will make image response unlikely. Furthermore, the greater frequency separation of such transmissions will aid the radio frequency attenuation of the image frequency response.

Direct intermediate frequency response Transmission of signals having the same frequency as the intermediate frequency through the radio frequency system is not believed to be a serious factor in frequency modulation receivers. Virtually any intermediate frequency chosen will be so far different from the radio frequency tune frequencies, that attenuation at the first detector input will be considerably better than in the case of 455 kilocycles on the standard amplitude modulation broadcast band. Nevertheless, all other things being equal, choice of a frequency to minimize direct intermediate frequency response is desirable.

.Combination of signal and oscillator har- Response from two stations separated by the intermediate frequency In this type of response one of the signals acts at the first detector as the local oscillator for the other signal, the first detector output being of the intermediate frequency. This type of response is likely to be serious in frequency modulation reception when a low intermediate frequency value is used, because stations may be allocated every 400 kilocycles in a given territory, and the percent frequency separation is small with respect to the signal frequency. This type response is particularly troublesome, because in localities where two strong signals exist separated by the intermediate frequency, they will be heard throughout the tuning range of the receiver.

Combination of signal and oscillator harmonics Most oscillators have appreciable harmonic content, so that if conditions exist under which signal harmonics occur in the first detector, spurious responses will result. The most common condition for signal harmonic production is when the signal exceeds the bias. Many frequency modulation receivers have not used automatic volume control, depending rather upon the limiter to maintain uniform signal amplitude at the second detector. Under such conditions, particularly with a-radio frequency amplifier present,

relatively low intensity signals will generate hara monics. Even with automatic volume control, harmonics may be generated by an undesired signal of high intensity when the receiver is tuned to a weaker desired signal. Whenever the difference frequency between the harmonics of signal and oscillator is equal to the intermediate frequency, a spurious response will occur.

Half intermediate frequency image Cross modulation has been found to exist in frequency modulation receivers just as in amplitude modulation receivers, and is due to the same cause. The side frequencies present in a frequency modulated carrier wave will be transferred from an interfering signal to a desired signal by non-linearity of the characteristic of the radio frequency amplifier, or converter. The application of automatic volume control usually accentuates cross modulation as it does in amplitude modulation. Similarly, cross modulation due to simultaneous existence of two or more strong fields impressed on a non-linear element external to, but in the vicinity of, the receiving antenna, will cause spurious responses. This effect has been found in localities having two strong broadcast band signals and may be expected in the frequency modulation range, also since simultaneous high field intensities will occur in that range in many locations. Since cross modulation takes place in the radio frequency system (or external to the receiver) it is not peculiar to superheterodyne receivers. Therefore, the choice of intermediate frequency does not influence this category of spurious response. The same precautions should be taken in frequency modulation as in amplitude modulation receivers to minimize cross modulation. The reduction in cross modulation by elimination of automatic volume control must be weighed against increase in signal harmonics which then occurs. In general, it is believed that signal and oscillator harmonics are more diflicult to deal with since remote cut-off tubes usually provide sufficient protection against cross modulation.

It may be stated that it is one of the main objects of our invention to provide a method of receiving frequency modulated carrier waves in the 42 to 50 megacycle band by the superheterodyne method with minimum spurious responses, and the method of reception essentially utilizing the step of reducing the center frequency of amplified frequency modulated carrier waves to an intermediate frequency value in excess of 8 megacycles.

Another important object of our invention is to provide a frequency modulation receiver, adapted to operate in the presently assigned 42 to 50 megacycle band, which employs good radio frequency selectivity prior to the converter with low radio frequency gain, automatic gain control being utilized on the radio frequency amplifier, and the intermediate frequency value being in excess of 8 megacycles.

Another object of our invention is to provide frequency modulation receivers of the superheterodyne type wherein an intermediate frequency of 8.26 megacycles is used where two tuned selector circuits precede the converter, and an intermediate frequency of either 11.45 or 13.5 meg-acycles is used in the case of a single tuned selector circuit preceding the converter.

Still other objects of our invention are to improve generally the efficiency of frequency modulation receiversc of the superheterodyne type, and more especially to provide a receiver of such type which is reliable in operation and economical in construction.

The novel features which we believe to be characteristic of our invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which we have indicated diagrammatically a circuit organization whereby our invention may be carried into effect.

In the drawings:

Fig. 1 schematically shows a frequency modulation receiver embodying the invention,

Fig. 2 shows graphically the number of spurious responses at different intermediate frequency values when operating the local oscillator at a frequency below tune frequency.

Fig. 3 shows similar curves for the case when the oscillator is higher than tune frequency,

Fig. 4 is a graphic analysis showing the relation between other spurious responses and intermediate frequency,

Fig. 5 is a graphic analysis showing the .probability of spurious responses due to different causes at different intermediate frequencies.

Referring now to the accompanying drawings, in Fig. 1 there is shown schematically a frequency modulation receiver of the superheterodyne type. The various networks are not shown in detail, since those skilled in the art are fully aware of the specific construction of the different portions of the receiving system. For example, there can be utilized a receiving system of the type described in our application Serial No. 319,- 830, filed February 20, 1940. In general, the numeral I denotes a signal collector which may be a di-pole or the well known type of grounded antenna. The collected frequency modulated carrier waves are applied to the tunable input circuit 2 of the first stage 3 of radio frequency amplification. As stated before, the carrier, or center, frequency of the applied wave will be located in the 42-50 megacycle band.

The input circuit 2 includes a variable tuning condenser, and the amplified carrier energy is transferred to the tunable input circuit 4 of the following converter stage. The circuit 4 is, also, provided with a variable tuning condenser, and the numeral 5 designates the local oscillator tank circuit of the converter 6. The variable tuning condensers of circuits 2, 3 and 5 are mechanically uni-controlled in any suitable manner. There will be developed in the tuned primary circuit 7 of the intermediate frequency transformer B frequency modulated carrier voltage whose center frequency is of the predetermined intermediate frequency value. The converter 6 may comprise a tube of the pentagrid converter type whereby the stage functions as a combined local oscillatorfirst detector. The tank circuit frequency may be higher or lower than the center frequency of the modulated carrier energy applied to the input circuit of the converter. For reasons to be explained in detail at a later point the intermediate frequency is chosen in excess of a critical value of 8 megacycles.

The secondary circuit 9 of transformer 8 is tuned to the operating intermediate frequency value, and the reactive coupling between the primary and secondary circuits I and 9 is chosen so that there is secured a wide band response at the intermediate frequency. It is thereby possible to pass a band of frequency modulated carrier energy some 200 kilocycles in width. Above the box HJ representing the intermediate frequency amplifier there is shown the type of response curve which is desired for the intermediate frequency transformers 3 and I I. It will be understood that the amplifiers I6 and I2 for the intermediate frequency energy Will be constructed in the same manner, and that the primary and secondary circuits of transformer II will be resonated to the operating intermediate frequency which is in excess of 8 megacycles.

An intermediate frequency transformer I3, also constructed in the same manner as transformers 8 and I I, impresses the amplified modulated carrier output of amplifier I2 upon the limiter I4. There may be used in the limiter stage the specific type of circuit disclosed in our aforesaid pending application. The function of the limiter is to eliminate amplitude variation in the carrier. Such amplitude variation arises by virtue of fading, noise impulses and passage of the modulated carrier wave through prior resonant circuits. The limiter is a form of easily overloaded amplifier, and operates with normal zero serting a load resistor I5 in the low potential side of the limiter grid circuit, the resistor being suitably by-passed for intermediate frequency currents. The direct current voltage developed across resistor I5 is applied over the path labeled AVC to the signal grids of the amplifiers 3, II] and I2. The automatic volume control lead includes sutiable filter resistors for suppressing pulsation frequency components in the automatic volume control bias. As the carrier amplitude varies at the collector I, the voltage developed across resistor I 5 is applied to the controlled amplifiers in a sense to vary the gain thereof so as to compensate for the carrier variation.

The limited intermediate frequency voltage is then impressed upon a discriminator-rectifier network ll which may assume any desired and well known construction. For example, the discriminator-rectifier network disclosed in our aforesaid application may be used to derive the audio modulating signals from the frequency modulated carrier wave. It is suflicient for the purposes of this application to point out that any network may be used at I! which has a characteristic such as is shown by the inclined S- shaped curve depicted above the rectangle H. The curve shown relates frequency swing of the carrier to rectified voltage output. The symbol f0 denotes the center frequency of the modulated carrier wave applied to the network ll. The extent of the carrier frequency swing of the modulated carrier wave corresponds to the amplitude variation of the modulating signal, while the frequencies of the modulation signal components correspond to the rate of deviation of the carrier. The output of network I! will develop the audio modulating signal. Of course, between the limiter I4 and the network I 'I there Will be transmitted the intermediate frequency energy which has its component frequencies distributed over a band 200 kilocycles in width.

For reasons specified at a later point it is desirable to employ in the amplifier 3 a tube of the variable mu, or remote cut-off, type. It is, also, desirable to have the circuits preceding the converter 6 of good selectivity. 'Ihe amplifier 3 may provide a relatively low gain. These various provisions are employed to minimize and substantially reduce spurious responses produced from different causes.

The value of intermediate frequency chosen has a large influence on the number of spurious responses produced. It is evident that, with a frequency range of 8 megacycles, an intermediate frequency higher than 4 megacycles Will eliminate image response due to frequency modulation stations. Similarly, an intermediate frequency higher than 8 megacycles will eliminate responses from two stations separated by the intermediate frequency, and an intermediate frequency higher than 16 megacycles will eliminate the half intermediate frequency image.

The effect of intermediate frequency on combinations of signal and oscillator harmonics is not as obvious, and detail analysis is required to determine them.

Let:

S=signal frequency O=oscillator frequency 2' =intermediate frequency T=tune frequency m=signal harmonic order 7L =oscillator harmonic order Then, any combination of signal and oscillator harmonics which results in the chosen intermediate frequency will produce a spurious response. It is assumed that the receiver can be tuned to any frequency between 42 and 50 megacycles. Also, only signals between 42 and 50 megacycles are considered. The analysis of the number of spurious responses produced is made without regard to the radio frequency selectivity. That is, for any signal between 42 and 50 megacycles any tune point in the same range may be chosen. It is, therefore, necessary to consider every possible tune frequency for every signal frequency in the range.

In determining the number of spurious responses, consideration of the relative magnitudes is reserved for later discussion. The lowest frequency channel is 42.1 megacycles, and the highest frequency 49.9 megacycles, a total of 40 in all. It is assumed that the total number of spurious responses from any cause is additive. That is, if for any given tune frequency there are three different signal frequencies which can cause a spurious response, three spurious responses are counted for that tune frequency. Thus, for some values of intermediate frequency, while there are only forty frequency modulation channels, the number of spurious responses is appreciably higher than forty. This form of analysis is more illuminating than indicating only the actual channels on which interference can occur without regard to the frequencies causing the interference. This is so because spurious responses will be caused mainly by strong signals, and a condition which will permit interference from any one of three undesired signals will be three times as liable to interference as one where there is possibility of spurious response from only one undesired signal. In the latter case the undesired signal might be too weak to cause a spurious response, whereas in the former case, if any one of the three undesired signals is strong enough, a spurious response will occur. It is, further, assumed that tuning is continuous between 42 and 50 megacycles, but that signals are confined to definite channels. Then, for spurious response to occur:

L 1) s+i(z -T if the oscillator is low er than the tune frequency.

m 1 9 j n S 7 n) 1 if the oscillator is higher than the tune frequency.

The method of analysis consists in assuming values of signal and oscillator harmonics, that is m and n, and, then, for each value of intermediate frequency determining the number of signal frequency channels between 42 and 50 megacycles which will satisfy the expression for all tune frequencies between 42 and 50 megacycles. It is apparent that consideration should be given to the oscillator higher than the tune frequency separately from the condition for oscillator lowor than the tune frequency, since the oscillator cannot change from one side to the other during the tuning process.

An example will illustrate the method of analysis. Assume the second harmonic of the signal and the second harmonic of the oscillator are to be considered, and that the intermediate frequency is 4 megacycles.

Then, Equation 1 becomes:

Then, S+6=T; or S+2=T each satisfy the conditions,

The lowest signal frequency is 42 megacycles, then the tune frequency must be 44 megacycles from equation S+2=T. The tune frequency can be any value up to 50 megacycles, so that any signal channel from 42 to 48 megacycles can produce spurious response under some tune condition. This is a range of 6 megacycles or 30 channels. Using the equation S+6=T, any signa1 frequency from 42 to 44 may produce a spurious response for some tune frequency between 42 and 50 megacycles or 10 signal channels. Therefore, for the second harmonic of signal and oscillator and an intermediate frequency of 4 megacycles there are a total of 40 possibilities of interferences, although there are only 30 channels between 42 and 48 megacycles, and 48 megacycles is the highest frequency producing interference under the assumed conditions. This simply means that for each of ten tune frequencies there are two signal frequencies which can produce interference.

To illustrate further, suppose the receiver is tuned to 49 megacycles, the oscillator then is at 45 megacycles and its second harmonic is megacycles. If the second harmonic of the signal is at either 86 or 94 megacycles, the resultant intermediate frequency is 4 megacycles and a spurious response occurs. That is, the fundamental of the signal may be either 43 or 47 megacycles. From inspection of the form of the expression, it may be seen that the number of channels subject to interference with the oscillator higher than the tune frequency is the same as for the oscillator lower. Furthermore, the form of the expression is a straight line with respect to intermediate frequency as a variable up to an intermediate frequency equal to megacycles, where it has a discontinuity and proceeds with a slope of one third its former value up to an intermediate frequency of megacycles, or 15 megacycles. For any intermediate frequency higher than 16 megacycles there will be no spurious response within the band from the second harmonic of signal and oscillator.

In like manner the condition for the third of the signal and the third of the oscillator, analyzed with respect to intermediate frequency, is of the form of two straight lines, the point of discontinuity occurring at the oscillator higher in frequency than the tune frequency, and then only for the range of intermediate frequency between 8.7 megacycles and 22 megacycles. For the combination of second harmonic of the signal with third harmonic of the oscillator, there will be no spurious responses with the oscillator higher than the tune frequency, and no spurious responses for an intermediate frequency lower than 6.50 megacycles nor higher than 33 megacycles. Similar analysis may be made for each order of signal and oscillator harmonic. A plot of the number of spurious responses as a function of intermediate frequency for the case where the oscillator frequency is lower than the tune frequency is shown in Fig. 2, and for the case where the oscillator frequency is higher than the tune frequency in Fig. 3. A plot of the number of spurious responses due to the image and to two stations separated by the intermediate frequency is shown in Fig. 4.

Note that these curves consider stations on each channel as having the ability to produce image responses, two stations separated by the intermediate frequency responses, and spurious responses caused by oscillator and signal harmonics. In actual practice, stations will not be so allocated in any given location, but the curves are valid in determining the best intermediate frequency to be used to reduce the number of responses, regardless of how many stations are assigned in any particular location, provided, of course, the stations are uniformly located in the band. The relative severity of spurious responses is difficult to evaluate exactly without data as to the magnitude of signal harmonic generation in the converter, which in turn depends upon the signal amplitude. However, an estimate may be made of the effect of oscillator harmonics, and instructive deductions made therefrom.

In a typical converter of the pentagrid type, the amplitude of oscillator harmonic has been found to be approximately 1/12 Where n is the order of the harmonic. That is, the second is quarter the fundamental, the third is one ninth the fundamental, and so forth. The conversion gain is not constant with respect to oscillator amplitude but reaches a maximum, usually a broad maximum (conversion gain substantially constant over an appreciable oscillator range) at some value of oscillator voltage. For values of oscillator voltage below this maximum the conversion is, to a first approximation, proportional to the oscillator voltage. Since the oscillator amplitude is 1/12 and the conversion proportional to amplitude, the conversion is approximately equal to 1/71 if the converter is operated so that the fundamental does not exceed the optimum. If the fundamental somewhat exceeds the optimum conversion point there will be no appreciable decrease in conversion, over the optimum value. But under these conditions the second harmonic may produce as much conversion gain as the fundamental. Consequently to minimize harmonic generation it is desirable to limit the oscillator injection to a value only suificient to develop maximum conversion. With the oscillator injection limited to such value, the probability of a given signal causing a spurious response is inversely proportional to the square of the oscillator harmonic order involved. This factor may be applied to the calculations for signal and oscillator harmonic combinations.

- Considering the image response, this is due to the fundamental of the signal and the oscillator, and so should not be decreased by any factor as are the harmonic combinations. In applying weighting to the case of two signals separated by the intermediate frequency, the oscillator is not involved, but the strength of the spurious response is proportional to the strength of the signals involved. This response will, in general, be less than that for the image. Accordingly, a factor of 0.5 was applied to the two signals separated by the intermediate frequency, giving it a weight between the image and harmonic combinations due to the second of the oscillator.

When weighting factors are applied it can no longer be said that the ordinates represent the number of spurious response signals. The ordinates in this case represent the probablity of spurious responses for various intermediate frequencies in a receiver having so selectivity ahead of the converter, with the probability for an intermediate frequency of one megacycle considered to be 100. The variation of likelihood of interference is shown in Fig. 5. This figure combines the weighted efiect of image, two signals separated by the intermediate frequency, and oscillator-signal harmonic combinations up to the fourth of both signal and oscillator. It illustrates that the probability of interference decreases rapidly with increasing intermediate frequency at first and then more slowly, and that over the entire range of intermediate frequency there is little choice between oscillator lower and oscillator higher conditions. It does not indicate definitely any best intermediate frequency, but does show that the intermediate frequency should be higher than 6 or 8 megacycles. In choosing an intermediate frequency from the range above 8 megacycles, other factors must be taken into account. In favor of a high intermediate frequency value are the factors of increased image ratio against signals outside the 42 to 50 megacycle range, the ability to secure the required band width with little or no added damping, and some slight decrease in the number of spurious responses due to signals within the band.

The types of signals, other than frequency modulation stations, likely to cause interference are those where the transmitter may be located in a populous district. These are amateurs, television and police signals mainly and possibly international broadcasting. In frequency modulation an innocuous frequency range of some 200 kilocycles is desired. Examination of frequency allocations shows that 8.26 megacycles is somewhat better than 8.25 megacycles over a 200 kilocycle width. At 8.26 megacycles are allocated ship telegraph and government services, neither likely to cause interference. On the score of image, however, with the oscillator higher, both the 5 meter amateur and the second television bands fall within the image range. With the oscillator lower, we find 10 meter amateur, police and government frequencies. It would appear, therefore, that this frequency, particularly with the oscillator lower, will be satisfactory in receivers having enough selectivity preceding the converter to insure good image ratio.

In receivers in the lower price classes where selectivity ahead of the converter is not high, it would seem desirable to operate with the oscillator lower than the tune frequency to avoid the television channels, and to use an intermediate frequency that would not permit interference from the amateur bands. This requires an intermediate frequency of over 11 megacycles. At 11.45 megacycles we find government, fixed and aviation allocations, and for the image, fixed, government and broadcast stations, so that this frequency is comparatively free from likelihood of spurious responses. Going to still higher frequencies, we find at 13.5 megacycles only fixed stations and for the image, broadcast stations. Therefore, this is another comparatively good frequency. It is not likely that frequencies appreciably higher than about 1 1 or megacycles will be useful, because of decreasing stability and gain limitation due to tube and circuit capacitances. There may be other frequencies which a close analysis of allocations would disclose to be desirable in the range 8 to 15 megacycles, but the three 8.26 megacycles, 11.45 megacycles and 13.5 megacycles appear to be as good as any, and preferable to most frequencies.

It has been pointed out that the use of automatic gain control is desirable on the radio frequency amplifier to decrease the possibility of generation of signal harmonics. However, the use of automatic gain control may result in some cross modulation, because of the possibility of strong signals on the frequency modulation band with a smaller percentage frequency separation than on the broadcast amplitude modulation band. Remote cut-off tubes, and good selectivity in the signal circuits, are preventives of cross modulation, Good signal circuit selectivity likewise serves to decrease the possibility of direct intermediate frequency and image responses, and to a lesser degree harmonic combinations. Harmonic combinations responses are less affected by radio frequency selectivity, because many of the combinations occur with small frequency separation of the causative signals.

In considering spurious responses, one char acteristic of frequency modulation not heretofore mentioned should be borne in mind, namely that such responses when they occur on the frequency of a desired signal do not cause an audible whistle as they do in amplitude modulation, but appear simply as cross-talk. Just what the ratio of desired to interfering signals at the input to the detector should be for substantial freedom from cross-talk is a moot point, opinions varying from 2:1 to 10:1 ratio. Tests made to determine this point tend to indicate that 2 to 1 may be satisfactory when the interfering signal is a single modulating tone (say 400 cycles), but that when the interfering signal carries program modulation the ratio should be nearer the 10 to 1 ratio.

It will, therefore, be seen that the spurious responses that may occur in frequency modulation receivers are of many types, but may be decreased by proper receiver design. The devices for substantially minimizing spurious responses are:

An intermediate frequency of 8.26 megacycles is recommended for receivers having two tuned selector circuits preceding the converter, and an intermediate frequency of either 11.45 or 13.5 megacycles for receivers having only a single selector circuit ahead of the converter.

While we have indicated and described a system for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of our invention, as set forth in the appended claims.

What we claim is:

1. In a superheterodyne type receiver, in combination, a radio frequency amplifier tunable over a desired frequency band, a tunable local oscillator, a converter, a fixedly tuned intermediate frequency amplifier having at least one tuned selector circuit, automatic gain control means for controlling the radio frequency amplifier, said latter amplifier having a relatively low gain, means providing relatively high selectivity to said converter, said intermediate frequency selector circuit being tuned to an operating frequency exceeding in value the extent of the radio frequency tunable range, thereby reducing to a minimum the spurious responses caused by combination of harmonics of signals within said tunable range with harmonics of the local oscillator.

2. In a superheterodyne type receiver, in combination, a radio frequency amplifier tunable over a desired frequency band of the order of 42 to 50 megacycles, a tunable local oscillator operating at its fundamental frequency, a converter to which is fed amplified signals and oscillations of said fundamental frequency, a fixedly tuned intermediate frequency amplifier having at least one tuned selector circuit coupled to said converter, automatic gain control means for controlling the radio frequency amplifier, said latter amplifier having a relatively low gain, means providing relatively high selectivity to said converter, said intermediate frequency selector circuit being tuned to an operating frequency exceeding in value the extent of said tunable radio frequency range thereby reducing to a minimum the spurious responses caused by combination of harmonics of signals within said tunable range with harmonics of the local oscillator.

3. In a superheterodyne receiver adapted to receive frequency modulated carrier waves, a carrier frequency amplifier including means for tuning it over a desired frequency band of the order of 42 to 50 megacycles, means for converting the amplified carrier energy to an intermediate frequency whose value is chosen from a range of the order of 8 to 15 megacycles, an intermediate frequency amplifier having at least one selector circuit, resonant to said chosen intermediate frequency, coupled to the converter, automatic gain control means for regulating the gain of the carrier amplifier, said carrier amplifier having a relatively low gain, and means providing relatively high selectivity to said converter.

DUDLEY E. FOSTER. JOHN A. RANKIN. 

